Taming a Thermal Expansion Relief Valve

[Bambie]

Relief valves installed on ASME VIII Div 1 vessels solely for thermal expansion relief may have significant inlet flow restriction so long as the required relief capacity is met and proper operation is not affected per Clause UG-135 (b)(1).

The tube side of a water heater operates at 900 psig with steam on the shell at 150 psig and 365 degF. The tubes are designed for 1000 psig corresponding to the heater pump shutoff head and the set pressure for the ¾”x1”nps thermal expansion relief valve.

Murphy designed an acoustic interaction between the channel volume, RV inlet line and relief valve dynamics that causes chattering, valve seat damage and chronic leakage after a brief pressure transient comes along.

I would like to introduce an orifice plate at the relief valve inlet flanged connection to mess with the acoustics and introduce vapour dampening. I would also like to install a take-off and normally closed valve between the relief valve inlet and orifice to permit manual re-seating once cycling is detected.

We have tried relief valves with reduced blow down and different seat materials with no luck.

Could this work Does anyone have experience with this configuration?

 

[Bambie]

Latexman and LittleInch,

Set pressure is confirmed through testing every five years.

The discharge line is 10 feet of 1"nps pipe that branches into a 4"nps common line that discharges to atmosphere. In addition, 6 inches downstream of the RV there is a 1"nps branch to a short stub with a 1/16" dia telltale hole.

I would prefer the simplicity of an orifice plate...mufflers are quite a bit more complicated to predict.

 

[Latexman]

Just curious. How serious is this issue? How often is it problem? Daily, weekly, monthly? Has a % downtime been assigned to it?

My muffler question was a stretch for me; I'm not that familiar with acoustics.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

Bambie, I still haven't seen in any response so far why you're not considering replacing the rather basic spring loaded valve you have with something a little more sophisticated which could easily solve all your problems (pilot drive relief valve) that both me and latexman think would help your situation?? You could probably replace this on line with sufficient safeguards at a small cost compared to the man-hours being eaten by something that might not work and which may require a shutdown to fit it.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Latexman]

There are modulating pilot operated PSVs (no pop action) that minimize system shock (i.e. less likely to add to the acoustic problem). Some even have internal damping especially for hydraulic hammer relief service (ISO-DOME). There may be other reasons you can't use a pilot operated PSV, they do have their own list od do's and don'ts, but I think it's worth a shot. I'd recommend you invite an Anderson Greenwood techie in for a look. This may not be your local sales rep! Lay it out for them and let them tell you what you need.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Bambie]

Latexman and LittleInch,

We have consulted with DRESSER technical support and they do not recommend a pilot operated relief valve in this application. These valves are larger, heavier, expensive and DRESSER are not confident that they will not chatter in this application.

I will follow-up on the Anderson Greenwood recommendation, thanks.

 

[Latexman]

I think AG's smallest pilot unit is 1" x 2" or 1.5" x 2". I recall they cater to a wider market than Dresser in pilot operated PSVs.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.